1. Field of the Invention
The present invention relates generally to devices that detect radiation emitted by sources in subterranean formations. More particularly, the present invention relates to devices that use a plurality of stationary gamma ray detectors to locate the source of gamma rays in a formation in the vicinity of a well bore. Still more particularly, the present invention relates to devices that utilize four gamma ray sondes to detect gamma rays from four discrete sectors of formation surrounding a well bore.
2. Description of the Related Art
The recovery of subterranean hydrocarbons such as oil and gas often involves an substantial investment in drill rig structures and expensive drilling operations. In order to maximize the return on both of these expenditures, rig operators may utilize one or more horizontal well bores that branch from a single vertical well bore. One situation suited to this technique involves a vertical well bore that is too distant from hydrocarbon deposits to permit efficient recovery. By drilling horizontally from a vertical well bore towards the hydrocarbon deposits, recovery is enhanced without the need for multiple drill rigs on the surface. Moreover, where a vertical well bore has penetrated into a productive hydrocarbon deposit, a horizontal well bore can improve the drainage of hydrocarbons into the well bore.
Effective horizontal drilling can often be accomplished by a steerable drilling assembly. Steerable drilling assemblies are discussed in pending application Ser. No. 09/081,961, filed May 20, 1998 and entitled "Well System," which is hereby incorporated by reference. When drilling horizontally, it is desirable to maintain the well bore in the pay zone, the formation containing hydrocarbons, as much as possible so as to maximize the recovery. However, pay zones may dip or divert in an unpredictable manner. Consequently, as a drilling assembly progresses through a pay zone, the drill bit may approach an adjacent nonproductive strata. The pay zone and adjacent strata define a bed boundary within which the operator may wish to confine drilling activity. Effective "steering" of the drilling assembly so as to maintain the bore within the pay zone is possible only where the operator has information relating to subterranean geology and ambient conditions.
Recently, the industry has developed a variety of devices and techniques to collect data during the drilling process. By collecting and processing data during the drilling process, the operator can make accurate modifications or corrections on-the-fly, as necessary, to optimize drilling operations. Designs for measuring conditions downhole and the movement and location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as "measurement-while-drilling" (MWD) techniques.
Gamma ray (GR) detectors are one type of tool that has been used in MWD and that can assist in maintaining a drilling assembly within the pay zone. Historically, gamma ray detectors have been used to either detect naturally occurring gamma rays in the formation or detect gamma rays emitted by an artificial source.
Passive gamma ray spectroscopy tools were developed in the mid-1970's to identify naturally-occurring radioactive elements emitting gamma rays in a formation. Gamma rays produced by different isotopes have characteristic energy spectra that can be used to identify the substance of the source emitting the gamma rays. In passive gamma ray logging, the naturally occurring radioactive isotopes, most commonly potassium, uranium and thorium, that are often present in a formation are the source of gamma rays sensed by the passive GR detectors. The incidence of gamma rays on the detectors, along with known information about the response of the tool, gives information about the source of the gamma rays, and thus gives information about the formation itself.
Prior art passive GR detectors include a scintillation crystal and a photomultiplier tube (PMT). The prior art GR detector is disposed in a pressure housing secured in a rotating portion of the drill string. Such a scintillation crystal reacts to incident gamma rays from a source at any angular location about the rotational axis of the drill string. While this would provide information as to the source of the gamma ray, this design would not provide some information as to the angular location of the gamma ray source. To obtain directional data, a GR shield may be installed in such a manner so as to limit the scintillation crystal's exposure to incident gamma rays to a specific azimuthal portion, or sector, of formation surrounding the prior art detector. A motor or other means rotates the directionally-sensitive OR detector that measures incident radiation in each of the angular directions as it rotates about the axis of the drill string. Thus, when a directional GR detector is facing the source of the gamma rays, the detector readings will be at their highest amplitude. Then as the directional GR detector rotates away from the isotopes, the signal decreases. The operator at the surface looks for the maximum in the gamma ray reading to determine the angular direction of the gamma ray source. The angular direction is thereby correlated to a useful azimuthal direction; that is, an angular direction relative to a known fixed point on a plane perpendicular to the axis of the drill string.
It is further known that a GR tool providing directional gamma ray information can be readily adapted to locate artificial sources of gamma rays in a formation. Specifically, GR tools can identify tracers, or radioactive isotopes, placed downhole. Thus, a tracer spectroscopy tool was developed to track the dispersion of frac fluids and solids from a wellbore and into the formation. Fracturing fluids and solids are pumped under high pressure into the formation surrounding the well bore to initiate and enlarge the fractures in the formation. These fractures then become packed with the solids, which are preferably a granular material that is highly permeable to the flow of hydrocarbons. This process assists the flow of hydrocarbons from the formation into the well bore. Using a directional GR tool together with radioactively tagged solids, the operator of the well can identify where the maximum frac fluids and solids flowed into the formation by analyzing the directional gamma ray signals from the formation.
Artificial gamma rays can also be generated by a source deployed in conjunction with gamma ray detector itself. Some of the gamma rays emitted by the source are reflected back by formation material and are detected by GR detectors. By analyzing the reflected gamma rays, the operator can ascertain the geological properties of the formation in the immediate vicinity of the GR detectors even in the absence of any naturally occurring gamma rays.
The utility of gamma ray detectors in analyzing the surrounding formation has led to their use in directing steerable drilling assemblies through the pay zone. Normally, gamma ray measurements in a particular pay zone are azimuthally uniform because such a pay zone consists mostly of one material, such as sandstone, throughout which the gamma ray emitting materials are more or less uniformly distributed. Strata material such as shale and sandstone have reasonably distinct levels of gamma ray emission counts. Thus, one method of maintaining a drilling path through the pay zone is to continually monitor the azimuthal directions of gamma ray emissions proximate to the steerable drilling assembly. As the drilling assembly nears a bed boundary, a directional GR detector will sense a variation in gamma ray measurements as a function of the azimuthal orientation. This is because the material in the adjacent strata emits gamma rays at a different rate from the pay zone. Once the variation is detected and its location is established, the operator can make corrections in accordance with known techniques to avoid exiting the pay zone.
Prior art directional GR detectors require a rotating section of drill string in order to sense gamma ray emissions from the 360 degree azimuth around the axis of the drill string. Steerable drilling assemblies can include rotating subs that could accommodate such GR detectors. However, some steerable drilling assemblies do not have a rotating drill string section suited for rotating directional GR detectors. For example, during "sliding" drilling, the portion of the drill string where the GR detector is housed is uphole of the mud motor and is not rotated. A motor can be used to rotate the section of drill string in which the prior art directional GR detector is mounted, but this string increases the power consumption due to the electrical or hydraulic power needed to rotate the detector and drill string section. Further, the steerable drilling assembly may not have a diameter large enough to accommodate a rotating section of drill string.
Thus, a need persists for a gamma ray detector that provides azimuthally sensitive gamma ray detection without unduly increasing power consumption and without limiting an operation to a specific type of steerable drilling assembly.